The two conventional common non-volatile data storage devices are: disk drives and solid state random access memories (RAMs). Disk drives are capable of inexpensively storing large amounts of data, i.e., greater than 100 GB. However, disk drives are inherently unreliable. A hard drive includes a fixed read/write head and a moving media upon which data is written. Devices with moving parts tend to wear out and fail. In addition, access times for hard drives are relatively slow. Solid state random access memories have faster access times and currently store data on the order of 1 GB (gigabyte) per device, but are relatively more expensive per storage unit, i.e., per 1 GB, compared to a disk drive.
The most common type of solid state RAM is Flash memory. Flash memory relies on a thin layer of polysilicon that is disposed in oxide below a transistor's on-off control gate. This layer of polysilicon is a floating gate, isolated by the insulator, typically silicon dioxide, from the control gate and the transistor channel. Flash memory is relatively slow, with writing times on the order of a microsecond. In addition, flash memory cells could begin to lose data after less than a million write cycles. While this may be adequate for some applications, flash memory cells would begin to fail rapidly if used constantly to write new data, such as in a computer's main memory. Further, the access time for flash memory is much too long for computer applications.
Another form of RAM is the ferroelectric RAM, or FRAM. FRAM stores data based on the direction that ferroelectric domains point. FRAM has access times much faster than Flash memory and consumes less energy than standard dynamic random access memory (DRAM). However, commercially available memory capacities are currently low, on the order of 0.25 MB (megabyte). In addition, memory storage in a FRAM relies on physically moving atoms, leading to eventual degradation of the medium and failure of the memory.
Yet another form of RAM is the Ovonic Unified Memory (OUM) that utilizes a material that alternates between crystalline and amorphous phases to store data. The material used in this application is a chalcogenide alloy. After the chalcogenide alloy experiences a heating and cooling cycle, it could be programmed to accept one of two stable phases: polycrystalline or amorphous. The variation in resistance of the two phases leads to the use of the chalcogenide alloy as memory storage. Data access time is on the order of 50 ns. However, the size of these memories is still small, on the order of 4 MB currently. In addition, OUM relies on physically changing a material from crystalline to amorphous; this physical change may likely cause the material to eventually degrade and fail.
Semiconductor magnetoresistive RAM (MRAM) stores data as direction of magnetic moment in a ferromagnetic material. Atoms in ferromagnetic materials respond to external magnetic fields, aligning their magnetic moments to the direction of the applied magnetic field. When the field is removed, the atoms' magnetic moments still remain aligned in the induced direction. A field applied in the opposite direction causes the atoms to realign themselves with the new direction. Typically, the magnetic moments of the atoms within a volume of the ferromagnetic material are aligned parallel to one another by a magnetic exchange interaction. These atoms then respond together, largely as one macro-magnetic moment, or magnetic domain, to the external magnetic field.
One approach to MRAM uses a magnetic tunneling junction as the memory cell. The magnetic tunneling junction comprises two layers of ferromagnetic material separated by a thin insulating material. The direction of the magnetic domains is fixed in one layer. In the second layer, the domain direction is allowed to move in response to an applied field. Consequently, the direction of the domains in the second layer can either be parallel or opposite to the first layer, allowing the storage of data in the form of ones and zeros. However, currently available MRAM can only store up to 1 Mb (megabit), much less than needed for most memory applications.
One alternative to current memory devices utilizes crosspoint memory arrays. In a crosspoint array, the storage medium is sandwiched between two sets of electrodes running in perpendicular directions. Thus, each element is at the intersection of one line (the word line) below and one line (the bit line) above and is addressed when a suitable voltage is applied between the two lines. The simplest memory scheme for such an element is a resistive switch that can be set to two or more resistance values by the application of a voltage or current pulse and then later read at a different voltage. The storage medium should exhibit a bi- or multi-stable behavior. In addition, the storage medium should switch at speeds fast enough to compete with hard drives at minimum. Further, the storage medium should retain its state for many years.
Viable candidates for application as memory technologies should be non-volatile unlike DRAM, and relatively inexpensive (compared to Flash) and with faster access times and greater mechanical reliability than disk drives. Crosspoint memory arrays promise to satisfy many of these requirements, and much effort is being applied in many research labs to develop a suitable storage medium. Reference is made to U.S. Pat. No. 6,055,180 to Gudesen et al.
Several candidates for such a bi-stable (or multi-stable) resistive switching element for use in crosspoint memory arrays have been described in the literature, but none has yet proved suitable for technological application. Resistive switches have been described by Y. Yang et al. PCT patent application No. WO02/37500 A1; and L. P. Ma et al. Appl. Phys. Lett., 80(16), 2997–2999, (2002), in which three layers (organic/metal/organic) are sandwiched between the electrodes. However, this structure does not consistently exhibit the bi-stable behavior required for crosspoint memory arrays nor are the characteristics of the device easily adjustable by rational design. In addition, both negative and positive voltage pulses are used to set the resistance of the device, increasing the complexity of the logic circuitry used to address the device in a crosspoint array.
Another memory device has been developed that utilizes a mechanism for multi-stable resistance behavior in which charge is trapped in a semi-conductive layer, is described in Simmons and Verdeber, Proc. Roy. Soc. A, 391, 77–102, (1967). The resulting electric field inhibits further injection at the electrode. In these metal-insulator-metal devices, an “electro-forming” step is required. This process comprises applying a relatively large voltage across the device that is believed to catastrophically destroy some fraction of the top electrode and deposit the metal as atoms and clusters of atoms into the insulating film. These atoms and clusters of atoms transport charge and act as charge storage centers. However, the electro-forming step is difficult to control and behavior of the device is not sufficiently predictable or reproducible for use in a memory device.
Another storage medium that comprises conjugated polymers, doped with ionic species to render it more or less conductive is described in Krieger et al., Proc. 6th Foresight Conf. on Molecular Electronics, (1998).
Another storage medium that utilizes a memory device comprising an alloy of silver with GeS or GeSe is described in Kozicki et al., Arizona State Univ. and Axon Corp., “Superlattices and Microstructures,” 27(5–6), 485–488, (2000). Electrochemical reduction of the silver creates metallic silver deposits that eventually percolate across the semi-conducting layer to form highly conducting pathways. This process can be reversed, re-oxidizing the silver and re-dissolving it in the semi-conductive matrix. Both of these memory media are inherently filamentary conductors where the current is concentrated in a few pathways connecting the electrodes, limiting the scalability of these devices to small dimensions. In addition, breakdown of the filaments would lead to catastrophic failure of the memory device.
What is therefore needed is a memory device that complements and/or replaces the existing product classes of DRAM, Flash, and hard drives. A storage device utilizing this memory device should be non-volatile, relatively inexpensive to produce in mass, and have greater reliability than hard drives. This memory device should have reproducible and predictable characteristics. The need for such a device has heretofore remained unsatisfied.